This invention relates to multistage hydraulic machines, and more particularly to an air exhausting device to be used in a multistage reversible pump-turbine having adjustable wicket gates.
In a case where a pump-turbine having a number of stages and return passages interconnecting these stages is started to be operated in a pumping mode of operation or in a synchronous condenser mode of operation, it is an ordinary practice to completely close wicket gates provided for the highest pressure stage of the pump-turbine, and to introduce compressed air into a chamber encasing the runner, thereby depressing the water level in the chamber for reducing the torque required for this mode of operation.
When the operation of the pump-turbine is transferred from the starting state to the actual pumping mode of operation, compressed air in the chamber is exhausted through an air exhausting system, while water is supplied from the draft tube of the pump-turbine to raise the water level in the chamber. Upon establishment of a predetermined water pressure (or priming pressure) at the delivery side of the pump-turbine, the wicket gates are opened thereby to initiate the pumping mode of operation.
In this case, however, the removal or exhaust of air from the lower pressure stage chamber connected with return passages is far more difficult than the removal of air from a chamber encasing the runner of a single stage pump-turbine or from the highest pressure stage of a multistage pump-turbine. Particularly, the removal of air from the lowest pressure stage of the multistage pump-turbine is most difficult, and an establishment of a technique overcoming such a difficulty has been urgently required.